Thrust coefficient

Thrust coefficient or ${\displaystyle c_{F}}$ (sometimes ${\displaystyle c_{\tau }}$) is a dimensionless number that measures the performance of a nozzle, most commonly in a rocket engine, independent of combustion performance. It is often used to compare the performance of different nozzle geometries. After combining it with characteristic velocity ${\displaystyle c^{*}}$, then an effective exhaust velocity ${\displaystyle c}$ and a specific impulse ${\displaystyle I_{sp}}$ can be found to characterize the overall efficiency of a rocket engine design.

The thrust coefficient characterizes the supersonic flow in the expansion section downstream of the nozzle throat, in contrast to characteristic velocity which characterizes the subsonic flow in the combustion chamber and contraction section upstream of the throat.